1. Field of the Invention
This invention relates to apparatus for recovering charged ions from solution and, more particularly, to apparatus for recovering such charged ions through a combination of electrodialysis and Donnan dialysis.
2. Description of the Prior Art
Various types of apparatus have been used in the past for separating and recovering charged ions from solution, such as the removal of plating metals from plating rinses. Some of the techniques, such as reverse osmosis, remove a large proportion of the metals from the rinsing baths. However reverse osmosis suffers from the disadvantage that the resulting solution containing the metal is at an insufficient concentration to be returned directly to the plating bath. Furthermore, since contaminating by-products as well as the charged ions are recovered, the plating bath may be poisoned by any attempt to reuse the recovered material.
Two processes which have recently been suggested for use in recovering charged ions from metal plating rinses are electrodialysis and Donnan dialysis. For example, U.S. Pat. No. 3,766,049 issued on Oct. 16, 1973, U.S. Pat. No. 3,909,381 issued Sept. 30, 1975, and U.S. Pat. No. 3,926,759, all incorporated herein by reference, describe recovery of metals from rinse solutions by the use of electrodialysis.
Furthermore it is known to separate charged ions by Donnan dialysis as described in U.S. Pat. No. 3,454,490, issued July 8, 1969, incorporated herein by reference. An article appearing in Dupont Innovation, vol. 4, no. 2, Winter 1973, pages 4-7 entitled "Metal Ion Recovery From Aqueous Industrial Wastes" describes the use of Donnan dialysis in several applications and furthermore suggests the use of such a system for the recovery of copper, nickel, cadmium, and chromium from electroplating wastes.
While each of these processes have been suggested for metal recovery, they have been discussed as alternatives to each other and each has disadvantages which have resulted in only a limited acceptance by the plating industry. Both electrodialysis and Donnan dialysis are based on the use of ion-exchange membranes which selectively pass ions of either positive or negative charge. In electrodialysis, the concentration of charged ions is usually conducted in an apparatus comprising an array of compartments arranged for parallel fluid flow partitioned by ion selective membranes. The membranes are arranged in a sequence, alternating between cation permeable and anion permeable membranes. A solution containing the charged ions to be recovered (the feed solution) is passed through the compartments in contact with the membranes and an electrical potential is applied across the system by means of electrodes contained in compartments at each end of a membrane stack. Under the influence of the electrical potential anions are forced in one direction through the anion permeable membrane while cations are simultaneously forced through the cation permeable membranes in the other direction, thus concentrating the charged ions in alternate compartments and diluting them in adjacent ones. On the other hand, Donnan dialysis employs only one type of ion-exchange membrane, a number of which may be arranged in a stack similar to the electrodialysis stack. In this case, the solution to be treated is introduced into alternating compartments and the adjacent compartments are fed a special stripping solution. The process acts to replace either the anions or cations in the feed solution (depending on the type of ion exchange membrane used) with the replacement anion or cation being supplied by the stripping solution. The driving force is the concentration difference between the similarly charged ions in the feed and stripping solutions and the process acts to replace the feed solution ions, in contrast to electrodialysis which removes the feed solution ions.
When recovering ions from rinsing baths, such as recirculating, dead, or reclaim solutions, it is preferred that the resulting solution of recovered ions be immediately returnable to the plating bath, and that the rinse solution be maintained at such a small remaining amount of dissolved ions that it can be either directly discharged into a stream or sewer, or in the alternative can be readily destroyed either directly or with minimum further treatment. Electrodialysis is most efficient at higher concentrations of metal per liter of solution. In fact, electrodialysis works reasonably well until a metal ion concentration of less than 250 ppm is reached, at which time a number of limiting factors arise. For electrodialysis to perform efficiently, the solution must contain a sufficient number of ions to maintain adequate conductivity. If the ion concentration (conductivity) becomes too low, the allowable current density gets unreasonably low, conductivity is lost, and separation ceases to take place. Any attempt to force an increase in the current density by increasing operating voltage results in polarization which splits water needlessly, wasting a great deal of energy.
Furthermore, the resulting change in hydrogen ion concentration may change the pH of the solution causing the ions being separated to precipitate, thereby plugging the membrane and rendering the unit useless. To control this problem the current density must be decreased as the concentration of charged ion in the feed solution decreases. As the current density is decreased, decreasing amounts of material are removed from the solution. The only way to compensate for this loss in recovery rate while still maintaining low current densities is to increase the membrane area. However, by doing this, the concentration of the solution produced will also be lower so that the recovered material may not be at a sufficient concentration to be immediately returned to the plating bath.
Alternatively, Donnan dialysis has a number of disadvantages which limit its use for recovery operations with solutions having higher concentrations of ions. The reasons are related to the mechanism by which Donnan dialysis works. Since the driving force is the concentration gradient between the stripping solution and the feed solution, the greater the concentration difference, the greater the rate of exchange or flux. For instance, if nickel is to be removed from a rinse solution, a potential replacement ion would be hydrogen obtained by using sulphuric acid as the strip solution. The higher the concentration of nickel ion in the feed solution, the higher the concentration of acid needed to increase the hydrogen ion concentration to a level where the net transfer of nickel ions from the feed solution to the strip solution is economically efficient and the higher the total amount of hydrogen ion necessary to replace the higher nickel content in the feed. For instance, to obtain a substantial flow when the concentration of nickel ion in the feed solution is about 250 ppm, a typical concentration suitable for electrodialysis, the stripping solution would probably have to be 2 N sulphuric acid and would require frequent replacement. As such high concentration, the properties of the exchange membranes also tend to break down allowing both anion and water transfer from the feed solution to the strip solution.